1. Field of the Invention
The present invention involves high numerical aperture (NA) lens assemblies. The present invention also involves a variable focus lens assembly including a first lens group and a second lens group, wherein a distance between the first lens group and the second lens group is adjustable to focus the variable focus lens assembly. The present invention may be used for examining body tissues, e.g., the cornea and the crystalline lens, at high NA such as at least about 0.65.
2. Discussion of Background
The cornea is a transparent tissue that not only allows transmission of light into the eye, but also provides most of the optical power for focusing images on the retina, the image sensing portion of the eye. Examination and treatment of the cornea is important because the regrowth of nerves after photorefractive keratectomy (PRK) and laser in situ keratectomy (LASIK) procedures is of interest in both research and clinical practice. Other procedures of more experimental nature can benefit from studies on the changes that are produced in nerve distribution, keratocyte density, haze, and epithelial cell regrowth (thickness and cell morphology).
The cornea is the most accessible component of the eye, and it has been studied extensively with microscopes of moderate and high resolution. In the living eye, the natural and frequent involuntary motions of the eye make it difficult to see and photograph fine details at high magnification. By the time the examiner identifies an image to be recorded, the eye may have moved in any or all of three dimensions. This problem is eased, but not completely solved, by having a distal end of the microscope objective suitably designed to contact the cornea, as disclosed in U.S. Pat. No. 5,359,373 to KOESTER et al. (hereinafter "KOESTER '373"), the disclosure of which is herein incorporated by reference in its entirety. The contact lens element serves to maintain the focus of the microscope at the same depth in the cornea, even when the eye rotates involuntarily. An additional advantage of contacting the cornea is that it establishes a plane of reference so that the depth of a region of interest can be determined by the setting of the microscope focus.
Another reason why the examination or treatment of the cornea is difficult is that the cornea is approximately 0.5 mm thick. In this regard, high numerical aperture (NA) objectives often have a limited range of high performance (diffraction limited) focal distance. For example, it is known that high NA objectives used to examine specimens on glass slides, with a cover glass between the specimen and the objective, can be used only with cover glasses having the thickness specified by the manufacturer. A few high NA objectives are made with a built-in adjustment that allows a small range of cover glass thicknesses to be utilized. Regarding examination and treatment of the cornea, high NA objectives tend to be diffraction limited over a limited range of about 0.1 mm of the cornea. Therefore, a lens assembly that is to have clinical application in examining or treating the cornea would preferably be able to obtain a sharp image over a substantial range of the thickness of the cornea.
Still another source of difficulty in examining or treating the cornea exists when the objects of interest lie at different depths in the cornea. In this regard, spherical and chromatic aberrations are generated at the interface between air and any medium having an index of refraction greater than 1.0. The magnitude of the spherical and chromatic aberrations increases with the thickness of the medium between the object to be examined and the surface (e.g., with the thickness of the cornea).
The difficulties involved in examining or treating the cornea also affect the examination or treatment of portions of the eye which are behind the cornea. For instance, the crystalline lens is located in humans approximately 4 mm behind the cornea. The crystalline lens provides additional optical power needed to focus the image precisely on the retina. In the crystalline lens, structures of interest include epithelial cells located close to the anterior and posterior surfaces of the lens, fibers within the interior of the lens, and suture lines where fibers from adjacent regions of the lens meet.
One reason why examination or treatment of the crystalline lens is difficult is that the position of the human crystalline lens varies from person to person. The position of the crystalline lens is defined by the anterior chamber (AC) depth and varies in the adult population over a range of at least 2.3 to 4.1 mm. BARRETT et al., Optom. Vis. Sci., Vol. 73, pp. 482-86 (1996), the disclosure of which is herein incorporated by reference in its entirety. During an examination or treatment of the crystalline lens, it is important to be able to focus at various depths in order to locate the structures of interest, and to determine the relative longitudinal positions of various structures. Further, because of this variety in depth of the crystalline lens, if the crystalline lens is examined by directly contacting the corneal surface with an objective lens surface, an objective system that is useful for one patient may need to be changed when a patient with a different crystalline lens depth is to be examined. It would be desirable if a large percentage of the adult population could be examined with one objective lens system, rather than having to change the objective lens system for nearly every patient. Therefore, an objective lens system that is to have clinical application in examining or treating the crystalline lens would preferably be able to obtain a sharp image over a substantial range of AC depths.
Another difficulty in examining or treating the regions in the eye posterior to the cornea, such as the crystalline lens and iris, results from the cornea being aspheric to the extent that the curvature of the cornea is greatest at the center and less toward the periphery. Most corneas also have some astigmatism, i.e., greater curvature in one azimuthal direction than in the perpendicular direction. The irregular shape of the cornea affects the path of light through the eye. This aberration in the light path may be corrected by spectacles or contact lenses.
Further, almost all corneas, when measured carefully, have some degree of irregular astigmatism. In irregular astigmatism, irregularities in the shape of the cornea cannot be fully corrected by spectacles.
To correct corneal astigmatism, a rigid, i.e., non-flexible, contact lens may be utilized, since it will generally have a layer of tears between the lens and the cornea, the thickness of which will vary from center to edge. The layer of tears improves the image which reaches the retina because the refractive index of the tears (1.336) is closer to the refractive index of the cornea (1.376) than the refractive index of air (1.0). Thus, when a rigid contact lens is used, variations in cornea topography have less effect on light rays than they would when the cornea is in air. As a result, the optical effects of corneal irregularity are significantly reduced. However, if the cornea astigmatism is sufficiently large, there will be residual astigmatism due in part to the fact that the refractive index of the tear layer does not exactly match that of the cornea. Further, there may be astigmatism in the crystalline lens or other irregularity in the crystalline lens.
As a result, for many years, rigid contact lenses have been used with patients having irregular corneas, particularly corneas having high levels of astigmatism. A rigid contact lens with a spherical back surface is prescribed, and the tear layer between the back surface and the cornea reduces the aberrations of the cornea by a factor of about: ##EQU1## That is, the aberrating effect of an irregular cornea is reduced to about 10% of that for the cornea in air.
Taking into consideration the known methods for correcting vision, there are several known techniques for facilitating the examination or treatment of the eye. Biomicroscopes, also known as slit lamps, are often used with a diagnostic contact lens which is hand-held against the cornea, utilizing a viscous liquid such as a methylcellulose solution to form an optical coupling to the cornea.
The use of diagnostic contact lenses reduces the aberrations that would be produced by the same cornea in air. The layer of methylcellulose solution reduces the effects of astigmatism because the refractive index of the methylcellulose solution (1.337) is closer to the refractive index of the cornea (1.376) than the refractive index of air (1.0). As a result, variations in cornea topography have less effect on light rays than they would when the cornea is in air.
Examples of diagnostic contact lenses which are used in conjunction with biomicroscopes include diagnostic contact lenses made by Ocular Instruments, Inc. and Volk Optical Co. These contact lenses are hand-held lenses with concave front surfaces to contact the cornea, generally used with a viscous liquid such as methylcellulose solution. Some of these lenses are gonio-lenses which include inclined mirror surfaces that allow examination of various regions of the retina and the region called the angle of the anterior chamber. FIG. 5 of WILENSKY, "Optics of Gonioscopy", Clinical Ophthalmology, Vol. 1 (1990), the disclosure of which is herein incorporated by reference in its entirety, shows how the gonio-lenses contact the eye, and how a ray of light travels from the angle recess of the eye to the mirror surface and then out of the lens.
KOESTER '373, which is briefly discussed above, involves a contact lens element that is moved relative to a high NA objective during focusing. Such objectives have been found to be valuable for studying cell structure, nerve configurations and the growth of nerves in the cornea, and identification of certain types of corneal diseases.
Microscope objective lenses with high NA are required for these applications since the resolution of the microscope increases in proportion to the NA of the objective. As noted above, in most objective lenses having high NA, the resolution is optimum over a very narrow range of focal distances. The apparatus of KOESTER '373 has an acceptable range of focus of about .+-.0.1 mm. In order to cover the 0.5 mm thickness of cornea, it is necessary to fabricate several contact lens elements to provide acceptable images over this range of focal depth.
KOESTER '373 discloses a contact lens element with a flat front surface for contacting the cornea to stabilize the longitudinal position of the cornea. Because the contact lens element flattens the portion of the cornea against which it is pressed, the contact lens element helps to reduce aberrations caused by the normal, unflattened shape of the cornea. KOESTER '373 discloses that small variations from flatness can be utilized, and discloses that if the surface is concave with a radius of curvature less than that of the cornea, it is possible to trap air bubbles in the tear layer between the contact lens element and the cornea, thereby disrupting the optical continuity of the system. KOESTER '373 also indicates that while a concave contact surface might have optical advantages, it could possibly cause a greater distortion of the cornea if it is not precisely aligned with the axis of the cornea.
The flat contact lens element of KOESTER '373, while useful for examining the cornea, is not suitable for examination of the crystalline lens and details at other depths within the eye. The flattening of the cornea has the effect of causing wrinkles, which show up as a corneal mosaic as discussed in AURAN et al., "Wide Field Scanning Slit in Vivo Confocal Microscopy of Flattening-Induced Corneal Bands and Ridges", Scanning, Vol. 16, pp. 182-86 (1994), the disclosure of which is herein incorporated by reference in its entirety. These wrinkles produce inhomogeneities in the optical path through the cornea and have been observed to result in a blurring of the retinal image during examination at high magnification.
KOESTER et al., "Clinical Microscopy of the Cornea Utilizing Optical Sectioning and a High-Numerical-Aperture Objective", J. Opt. Soc. Am. A, Vol. 10, No. 7 (July 1993), the disclosure of which is herein incorporated by reference in its entirety, discloses contact lens elements similar to those disclosed in KOESTER '373. This article discloses that a slightly concave surface can be used for the contact lens element, as long as the radius of curvature is greater than that of the cornea, to reduce the possibility of trapping air bubbles between the contact lens element and the cornea.
In the examination of the cornea, the Tomey confocal microscope is finding good acceptance. The Tandem Scanning confocal system is now being offered by Advanced Scanning Limited, New Orleans, La. The Tandem Scanning confocal system, which has a NA of about 0.60, involves a lens having positive optical power designed to contact the cornea and an internal lens which moves relative to the lens in contact with the cornea. The Tandem Scanning confocal system, however, is designed to investigate the cornea.
U.S. Pat. No. 4,666,456 to SHIMIZU et al., the disclosure of which is herein incorporated by reference in its entirety, discloses a construction involving three lens groups in which the middle lens group is moved along the optical axis to correct deterioration of image performance due to various thicknesses of parallel glass plates, e.g., microscope cover glasses. The first lens group has positive refractive power while the second and third lens groups have negative refractive power. The function of the second lens group is also to create negative spherical aberration of variable magnitude.
U.S. Pat. No. 576,896 to RUDOLPH, the disclosure of which is herein incorporated by reference in its entirety, discloses that two glasses having similar indices of refraction and different dispersions may be used to correct chromatic aberrations.
KINGSLAKE, Lens Design Fundamentals, pp. 105-07 (1978), the disclosure of which is herein incorporated by reference in its entirety, shows that in certain lens systems, as the focal distance is increased, spherical aberration passes through a zero point, increases slowly and reaches a maximum point. KINGSLAKE also discloses a maximum positive value of spherical aberration when the radius R is about: EQU R=1.2*r.sub.apl (eq. 1)
where r.sub.apl =the aplanatic radius for the particular index of the glass : EQU r.sub.apl =L/(1+n) (eq. 2)
where n=index of refraction of the glass, and L=the distance from the surface to the point of convergence of a bundle of rays entering the surface.
With the above in mind, there exists a need for a high NA lens assembly which is diffraction limited and which can focus over a broad range. For instance, such a lens assembly could be used to examine or treat transparent or semi-transparent materials including body tissues such as the cornea or the crystalline lens.